A variable capacitor, referred to as a varactor, is one type of circuit element commonly used in RF and microwave circuits. A conventional varactor is a semi-conductor diode that exhibits a change in capacitance in response to a change in applied voltage.
Circuit elements conventionally used in RF and microwave circuits, including varactors, switches, inductors and filters, cannot be integrated into a common assembly with the same fabrication technology, necessitating that discrete circuit elements be mounted on a secondary substrate, e.g. a planar piece of ceramic suitable for high frequency applications. This adds to the cost and size of the RF or microwave circuit, and can impair performance.